X-ray radiation may be generated by bombardment of an anode with an electron beam emerging from a cathode. The cathode and the anode are in this case arranged in a vacuum housing of an X-ray tube. X-ray tubes may be provided with a rotary anode that rotates away on impingement of the electron beam in order to avoid a focal spot that is stationary with respect to the anode. The focal spot, (i.e., the point at which the electron beam impinges on the anode surface), is shifted along a circular path over the anode surface from the point of view of a coordinate system rotating with the rotary anode. As a result, the lost heat generated on impingement of the electron beam is distributed comparatively uniformly over the anode surface, as a result of which possible overheating of the material at the focal spot is counteracted.
The X-ray rotary anode of known X-ray tubes is driven by an asynchronous motor, which is fed by an inverter. The rotor of the asynchronous motor is coupled to the rotary anode and is located within the vacuum envelope of the X-ray tube. Such a drive apparatus is disclosed, for example, in DE 197 52 114 A1.
Three windings offset with respect to one another through 120° are arranged in the stator of the asynchronous motor, for example. The rotor includes a magnetic return path, and an electrically conductive material, which is arranged as a cage or bell. The magnetic return path may also be embodied fixedly. If a sinusoidal electric current is flowing in the windings of the stator and there is a phase shift of 120° between the currents, a rotating magnetic field is formed in the stator of the motor. This magnetic field passes through the rotor. The rotating magnetic field induces an electric voltage in the conductors of the rotor. Since the conductors are short-circuited owing to their embodiment as a cage, the induced voltage brings about a current flow in the rotor. The rotor current builds up a dedicated magnetic field that interacts with the rotating magnetic field of the stator. A torque acts on the rotor, as a result of which the rotor implements a rotary movement and follows the rotation of the stator field.
The rotor, however, does not follow the rotating magnetic stator field synchronously, but rotates at a lower speed. The relative movement of the rotor and the stator field is necessary since only then is a current flow induced in the rotor and may the rotor build up its dedicated magnetic field. The rotor therefore rotates “asynchronously” with respect to the stator field. There is a slip between the frequency of the stator field and the rotational frequency of the rotor. The magnitude of the slip is dependent on the loading and on the size of the air gap between the rotor and the stator. During no-load operation, the slip is only low.
The air gap between the windings of the stator and the rotor is very small in the case of conventional asynchronous motors. In the case of an X-ray tube, however, a mechanically larger air gap is desired since there is a tube sleeve between the stator and the rotor, which tube sleeve provides the tube vacuum. If the rotor is additionally also at a high-voltage potential, an even larger distance needs to be maintained with respect to the stator in order to provide electrical insulation. The large air gap between the rotor and the stator has the effect that the magnetic flux density of the stator is low at the location of the rotor. The available torque is low since the Lorentz force on the rotor is low in comparison with a conventional asynchronous motor.
Also problematic are the eddy currents in the rotor of an asynchronous machine since they generate additional lost heat in the X-ray tube. The heat of the rotor needs to be dissipated, which is difficult as a result of the prevailing vacuum. In addition, the heating results in an increase in the resistivity of the rotor material, as a result of which the torque on the rotor is additionally reduced.
In principle, an asynchronous machine with a large air gap has a power factor of less than 0.5. That is to say that the motor draws a large quantity of reactive power, as a result of which the current amplitude becomes very high. DE 10 2011 077 746 A1 proposes providing a rotary anode of an X-ray tube with a synchronous drive. A rotor including a permanently magnetic material is used in place of a squirrel-cage rotor of an asynchronous drive. If the rotor is magnetized, the permanent magnets generate a standing magnetic field with respect to the rotor. The rotor rotates synchronously with a rotating magnetic field generated by a stator.
DE 10 2011 077 746 A1 discloses a rotary anode for an X-ray tube including a rotor for driving the rotary anode, wherein a magnetic field of a stator winding exerts a torque on at least one permanent magnet arranged in the rotor. The advantage of the synchronous drive includes that eddy current losses are minimized in the rotor and the power factor cos φ tends towards 1. As a result a rotary anode may be driven more efficiently.
FIG. 1 depicts a longitudinal section through the X-ray tube including a synchronous drive in accordance with DE 10 2011 077 746 A1. In an evacuated tube sleeve 2 of an X-ray tube 1, there is an electron-emitting cathode 3 and a rotary anode 4 opposite said cathode. The rotary anode 4 includes an anode plate 41, which is connected to a rotor 43 of an electric motor by a shaft 42. Magnetized permanent magnets 44 are arranged in the rotor material 45 of the rotor 43 and generate a magnetic field that rotates along with the rotor 43.
Outside the tube sleeve 2, a stator 5 surrounds the tube sleeve 2 in the direct vicinity of the rotor 4. The stator 5 generates, with its stator windings 51 through which current is flowing, a magnetic field rotating about the tube sleeve 2. That stator 5 also exerts a torque on the rotor 43 and therefore causes the rotary anode 4 to rotate synchronously corresponding to the remarks made in respect of FIG. 1. The stator windings 51 are arranged in a laminate stack 52.
The electron beam 6 emitted by the cathode is accelerated towards the anode plate 41 and, on impingement on the anode plate 41, generates X-ray radiation 7 owing to deceleration, which X-ray radiation leaves the X-ray tube 1 through a beam window 8 in the tube sleeve 2.
Temperatures of over 300° C. occurring during operation of the X-ray tube and temperatures during manufacture of the X-ray tube of up to 600° C. may be problematic for the permanent magnet of the rotor.
U.S. Pat. No. 4,322,624 A discloses a rotary anode including an electric-motor rotary anode drive including a coil and a permanent magnet.
WO 2010/136325 A2 discloses an axial hybrid bearing that includes a permanently magnetic bearing for generating a repulsive force and an electromagnetic part for generating an attractive force.